The strength of the resonant opacity is proportional to the number of absorbers, calculated from the Saha equation (e.g. [1]}). Only for the highest values of \(\dot{M}/f_{\mathrm {fill}}\)  and \(M_{\mathrm {p}}\)  is the gas at least partially ionised when reaching the shock; for most \((\dot{M},M_{\mathrm {p}})\)  combinations, the hydrogen is atomic (see black dotted lines in Figure REF ). At low \(\dot{M}/f_{\mathrm {fill}}\)  the hydrogen is molecular.
